1. Field of the Invention
This invention relates to methods of manufacturing open-celled ceramic articles and, more specifically, to methods of manufacturing high-strength silicon nitride heat exchangers for use in gas turbines and the like.
This invention is an improvement over U.S. Pat. No. 3,940,301, entitled "Method of Manufacturing an Open Cellular Article" issued to Straw et al and owned by the assignee of the present application, the details of which are herein incorporated by reference.
Straw discloses a method of manufacturing a ceramic article with accurately shaped openings, such as a cross-flow heat exchanger. According to Straw's method, a plurality of wall-forming members are formed from a ceramic-based mixture and are placed in edge contact with each other to form the desired shape of the final product.
Partially disintegratable supports are placed between the walls and the resulting structure is exposed to heat and pressure within a die. The structure undergoes an initial hot press wherein the temperature is raised to 1200.degree. F. and pressure up to 500 psi is applied. This is followed by the final hot-pressing step wherein the temperature is raised to between 2500.degree. F. and 2950.degree. F., and pressure between 2000 and 2500 psi is applied and maintained for a period of up to approximately four hours.
The disintegratable portion of the supports and the walls is removed during the initial hot press step. Upon removal of the remainder of the supports following the final hot-press, a plurality of accurately shaped openings in the article is produced.
The walls of Straw's article are composed essentially of a mixture of an elastomeric binder, such as EPDM (ethylene propylene diene modified) rubber, and small particles of a suitable refractory material, preferably silicon nitride. The supports comprise a mixture of graphite powder filler and an elastomeric binder, such as EPDM rubber.
The use of passage-forming supports described above substantially eliminates the tendency of the walls to sag prior to the heating and hardening stages. This greatly facilitates the formation of intricate internal passages in the fired article. Further, the supports allow the preform to be extensively handled without damage.
One application of the type of heat exchanger produced according to the above method is the use thereof in gas turbines. Such turbines may operate at temperatures up to 2500.degree. F. Operation at such temperatures may require heat exchangers which can withstand the stresses which accompany thermal cycling up to temperatures of 2160.degree. F., as during start-up, for example. Heat exchangers operating at such temperatures require walls whose bend strength substantially exceeds 100,000 psi.
One disadvantage of the method of manufacture described above is the limited wall strength (or bend strength) of the finished product. The walls of heat exchangers made according to the above method exhibit bend strength in the range of 40,000 to 60,000 psi. This limited strength is due to chemical reactions occurring at approximately 2850.degree. F. to 2900.degree. F. between certain components of the wall supports and walls. The reactions, described below, remove silicon nitride from the walls, thereby weakening them.
Due to oxidation, all silicon nitride [Si.sub.3 N.sub.4 ] particles in the walls are coated with silica [SiO.sub.2 ]. At about 2600.degree. F., the silica breaks down to form silicon monoxide and oxygen according to the following: EQU 2 SiO.sub.2.sup. 2600.sup..degree. F. 2 SiO[g ] + O.sub.2 ( 1)
at about 2850.degree. F., it is believed that the gaseous silicon monoxide reacts with the graphite present in the supports to form silicon carbide [SiC], carbon monoxide and carbon dioxide: EQU 3 SiO+ 5C.sup. 2850.sup..degree. F. 3 SiC+ CO+ CO.sub.2 ( 2)
the carbon monoxide and carbon dioxide then react with the silicon nitride particles present in the walls to form more silicon carbide, nitrogen and silica: EQU 2 CO+ 3CO.sub.2 + 3 Si.sub.3 N.sub.4 .fwdarw. 5 SiC+6N.sub.2 +4SiO.sub.2 ( 3)
the silica produced in equation (3) reacts according to equation (1) in continuation of the cycle.
The above reactions are detrimental in that the silicon nitride in the walls is replaced by silicon carbide, which imparts relatively low strength to the walls. It is possible to avoid this detrimental substitution by limiting the hot press temperature to values slightly less than 2850.degree. F., at which point silicon carbide is formed according to equation (2). However, it has been determined that wall bend strength is positively related to hot press temperature, and hot press temperatures up to 2850.degree. F. will yield bend strengths of only 40,000-60,000 psi.
It has been found that high temperature (i.e., above 3,000.degree. F.) post-manufacture heat treatment of articles produced according to the Straw patent could result in increased wall strength. Such treatment may be effected with or without the application of pressure, but substantially all traces of the graphite filler material must be removed prior to the treatment so as to avoid the detrimental reactions described above. It may be understood that such heat treatment is a time-consuming and expensive step.